In wireless communication systems, the frequency spectrum is a scarce resource that calls for efficient use. Such, efficiency was first obtained by dividing a geographic area into smaller regions or cells and to assign a limited number of frequency channels to each cell.
Depending upon the access technique, the frequency channels might or might not be re-used in adjacent cells. For frequency division multiple access (FDMA) systems, such as is disclosed in the Global System for Mobile Communications (GSM) standard, it is preferred that adjacent cells do not share the same frequency channels, so as to mitigate any co-channel interference. In such systems, in order to provide a minimum quality of service, which is related to signal to interference plus noise ratio (SINR), a minimum distance is maintained between cells that use the same frequency channels. Thus, only a subset of the available frequency bandwidth is used, but in a plurality of cells of a cellular network.
In order to further increase capacity, the paradigm that a frequency channel is allocated to a single user has also been supplanted, such as by the time division multiple access (TDMA) technique employed in the GSM standard, wherein a particular user transmits and/or receives in a limited number of periodic time intervals or packets or time slots. In the GSM system, the time bandwidth is divided into frames of eight packets. Thus, up to 8 users could share a single frequency channel within a cell without risk of interference. Accordingly, the maximum number of users per cell that could be simultaneously connected to a base transceiver station (BTS) servicing the cell is the product of the number of frequency channels allocated to the cell and the number of time slots per channel.
Typically, communications between a user or subscriber station (SS) and the base transceiver station takes place across two links, designated the forward or downlink (DL) from the base transceiver station to the subscriber station and reverse or uplink (UL) from the subscriber station to the base transceiver station. The allocated frequency channels per cell could be used by both links, for example, in time division duplex (TDD) systems. Alternatively, one could allocate all of the channels to a single link. In such a case, for instance in frequency division duplex (FDD) systems found in the GSM standard, either the number of channels would have to be duplicated or half of the allocated channels would be used for the downlink and half of them would be used for the uplink, effectively halving the user capacity.
From an implementation point of view, two antennas could be used in FDD systems, one of which could be assigned to the receive function and the other to the transmit function. Alternatively, a single antenna could be used for both transmit and receive functions. In such a case, transmit and receive chains would be separated by some means, such as by a duplexer and accompanying filters.
In early embodiments of wireless communications systems, antennas were designed with a geographically constant radiation pattern intended to cover the associated cell region. Depending upon the cell size, the antenna's transmit power would be optimized to cover the entirety of the cell, taking into account such parameters as propagation environment, transmit power and losses in the transmit chain, all of which potentially affect the maximum reach of the antenna. For simplicity, initial cell antennas were omni-directional and transmitted constant power in all directions from the antenna location in the centre of the cell.
Later developments included sectorization as a mechanism for further capacity increase. In sectorization approaches, the antenna was made directional and its beam-width was limited, so that the cell size or coverage area was limited not only by the maximum reach of the antenna but also by the angular space covered by it. Conceptually, if an omni-directional antenna was represented by a circular disk, a sectorized antenna could be represented by a limited pie-shaped slice of the disk. While in theory, an antenna could be designed to have any desired beam width, typically, beam widths have tended to be either 33°, 45°, 65°, 85°, 90° or 105°.
Currently, most implementations deploy tri-sectorization in which a cell is split into 3 sectors of approximately 120° each. An antenna having a 90° beam width services each sector and the three antennas are co-located. Thus, the real estate cost of such implementations is reduced because a single site supports three sectors. As well, because the sectors are outward facing, co-channel interference is minimized. The sectors however, are not necessarily equal in size. Rather the sector design is primarily predicated upon user distribution.
Were interference the only factor to be considered in network design, higher order sectorization would be preferable, with a practical upper bound being hex-sectorization. However, the number of users being in handover increases dramatically as the sector width decreases. Furthermore, in older implementations, where cells are closely packed, it is difficult to increase the level of sectorization.
Thus, as user capacity levels become saturated, other capacity increase methodologies have gained prominence. Chief among these methodologies is spatial filtering, or beamforming, in which a narrow beam is pointed to a particular user and adapted to track the geographical position of the user. In beamforming systems, multiple copies of the signal are received through multiple antenna elements and combined in such a way as to increase the signal to noise ratio (SNR) or the SINR.
In some beamforming systems, rather than directing energy to where the user is located, nulls in the radiation pattern generated by an antenna are systematically generated where co-channel interferers are located.
The location of such interferers may be identified because in the uplink direction, the data packet contains information known to the base transceiver station receiver and used by it to generate a vector of weights (magnitudes, phases or magnitudes and phases) that combine the antenna signals so as to form a beam directed toward the user or a null directed toward interferers. For example, in the GSM standard, provision is made for the inclusion within the data packet of a known training sequence that identifies the user and provides opportunity for the propagation environment to be ascertained.
In TDD systems, because the subscriber station communicates with the base transceiver station in both directions along the same channel frequency, the vector of weights determined in the uplink direction can be reused in the downlink direction, assuming that the propagation environment parameters are relatively constant across the short interval between the uplink and downlink communications.
By contrast, in FDD systems, the channel parameters of the downlink direction are uncorrelated with those of the uplink direction. Thus, in such systems, the base transceiver station transmitter predicts the channel parameters for the downlink direction based on certain knowledge of the channel parameters of the uplink direction such as the direction of arrival (DoA) and averaged powers, again on the assumption that these parameters remain relatively constant and are independent of the particular channel frequency used.
Thus, the base transceiver station transmitter could build weight vectors for the antenna array in order to produce appropriate beams and/or nulls in accordance with the channel invariant parameters from the uplink. Such null steering and beamforming approaches are, in theory, effective in situations where the desired and interfering signals are sufficiently separated in angular space. The amount of separation called for is related to the size of the antenna array.
Nevertheless, it is recognized by those having ordinary skill in this art that such approaches, especially null steering approaches, suffer from robustness issues because of the relatively narrow nulls or wide beams that are generated. For example, sometimes the interferer is so close in angular direction to the user that broad nulls will not be generated in the direction of the interferer for fear of dropping communications with the user.
Where relatively narrow nulls are created, a small error in the array calibration or in the estimated position of the interferer, such as by motion of the interferer in angular space, will result in the null being directed at an angle away from the appropriate location. If the calibration error is small, the impact on the interference cancellation ability of the beamforming system for forward link will be less significant. However, as the error increases, whatever interference cancellation ability is obtained may be significantly attenuated or wiped out entirely.
Thus, some approaches implement null broadening schemes so as to mitigate the detrimental impact of calibration error on the performance of the beamforming system, to ensure the actual performance approximates, as closely as possible, the theoretical performance gains available with null steering techniques. In other words, the objective is to avoid dramatic degradation in the performance in such situations.
Those having ordinary skill in the art will recognize that null broadening techniques are relatively simple with a large number of elements in the antenna array. However, in practical implementations, in order to keep system costs at a moderate level, the number of elements is typically kept small. In such situations, the ability to provide null broadening is compromised.
For example, null broadening is typically considered a constrained optimization problem, which involves iteration over a long period of time in order to converge to a solution. To reduce the complexity, certain of the constraints may be alleviated. For example, the number of interferers could be reduced or combined in a desired broadened null location in order to simplify the iterative algorithm.
Accordingly, it is desirable to provide a means of broadening nulls with a limited number of elements in the antenna array.